1. Field of the Invention
This invention relates generally to the field of stents for use primarily in ducts and vessels of the body, and more particularly, to the area of expandable stents which expand and/or reconform after implantation in the body.
2. Art Background
A number of biomedical devices have been used for the stenting of body passages that have become blocked or are otherwise damaged, or weakened. Expanding stents seem the most promising for this purpose because of the advantages provided by insertion of a small diameter tube-like device and the ultimate enlargement of the diameter to accommodate the flow of body fluids or food and the increase in time prior to occlusion of the stent.
There are a number of different types of devices for stenting of body passages that may be classified into two general categories: namely, expandable meshes which includes self-expanding types and non-self-expanding types, and non-expandable stents which are typically made of plastic or polymeric material. The self-expanding meshes can be made of a material, such as Nitinol, which changes configuration upon heating to body temperature. Other self-expanding meshes are made of resilient mesh material which can be flexed down into a small diameter tube and held in place in such a configuration until it is released, at which time it expands to a larger diameter configuration. The non-self-expanding meshes are expanded by use of an inflatable balloon which is placed inside the mesh in a small diameter configuration, and then inflated, thereby expanding the mesh to a larger diameter configuration. The balloon is then deflated for removal, leaving the mesh in its expanded configuration.
The expandable metal mesh stents are particularly advantageous in that they can increase in size up to 36 Fr. However, the metal meshes are difficult to remove since tissue in-growth occurs over time. A number of other problems, for example, mucosal hyperplasia, fungal proliferation, or neoplasm also are common with expandable metal mesh stents.
The expandable mesh stents can be made in a variety of materials such as, for example, stainless steel. The expandable mesh stents can be made in a variety of configurations such as a coiled spring, a variety of zig-zag patterns including various stents knows as "Z" stents, braided filament, and other collapsible configurations. One type of expandable stent comprises a cylindrical member having a slit cut along its length, so that the edges along the length can overlap to allow for compression to a reduced size. A further configuration which may be described as a perforated tube comprises a generally rigid tube with openings cut therein to allow for radial expansion under force of an expansion balloon or by expansion due to heating. The expandable mesh stents can be braided, woven, knitted, formed, molded, machined or made by other methods known in the art. Various mesh designs are disclosed, for example, in U.S. Pat. Nos. 4,512,338, 4,503,569, 4,922,905, 4,733,665, 4,950,227, 5,089,006 and 5,061,275. Other materials, configurations and methods of manufacture in addition to those described above are known.
As used herein, the term "expandable mesh" is meant to include, without limitation, self-expanding and non-self-expanding configurations made of any generally rigid or springy material which when expanded have an open network or arrangement which would otherwise allow tissue in-growth, and would not otherwise prevent fluid flow through its walls. Several of these prior art mesh stents have been utilized with a polymeric sheath or cover; however, since these sheaths must be stretched to increase in size, they exert a force that resists expansion, which tends to limit the final expanded size of the mesh. Additionally, this resistance may make expansion more problematic. Alternatively, the sheath may be folded or bundled over the mesh when it is compressed, so that no force is exerted upon expansion. However, this method increases the size of the compressed stent, so that a larger size catheter is required for a given size stent.
The plastic or polymeric stents exist only in smaller sizes because of the difficulty of inserting a larger catheter through the channel of existing scopes or in narrow body passages, such as veins or arteries, bile ducts, the esophagus, prostate, and the like. A disadvantage of the plastic stents is that as a result of the smaller size of the lumen, the plastic stents occlude faster than the expandable stents. One advantage of plastic or polymeric stents is that their closed structure prevents tissue in-growth which therefore makes it easier to remove, and less likely to be occluded by tissue in-growth from the outside. Some plastic or polymeric stents, as mentioned above, are simply small tubes.
With the current self-expandable mesh stents, the tubular body may comprise a flexible material where the diameter can be changed between a small diameter for insertion, and a final, expanded diameter after placement allowing for fluid flow therethrough. In some cases the self-expanding device assumes its final, expanded configuration when in an unrestricted condition, free of external forces in the radial direction. These type of devices must be compressed and loaded into a sheath that restrains the device radially for insertion into the body via a catheter. After insertion, the sheath or loading system may be removed to allow for expansion. After removal of the sheath, the device expands to its final diameter under spring-like forces resulting from the compression. In the case of heat sensitive materials, the stent takes on its expanded shape after it is heated above its transition temperature. With non-self-expanding mesh stents, upon expansion by the use of, for example, an expansion balloon, the final expanded configuration is achieved. Additionally, other methods of expansion and contraction are known.
What is needed is an expandable stent that allows for easy insertion and that increases in size to increase the time for occlusion to occur. The expandable stent should eliminate or minimize tissue in-growth without requiring a covering which either resists expansion or which must be folded or bundled when the stent is in the small radius, non-expanded condition. Further, it is desirable in some situations that the expandable stent does not require a loading sheath, so that a smaller sized catheter can be used to insert the stent into the body.